And once again we have something we haven't seen in quite a while! The BGM for the Crystal Empire episodes was, as always, totally epic. I think it was a wishbone episode when I was ~10 years old that taught me the importance of music in making a scene really "pop out", and this show does it amazingly well.
Anyway, I'll quick blabbering. Have a list:
Legend of the Crystal Empire
The Failure Song
King Sombra's Return
The Ballad of the Crystal Empire
Restoration of the Crystal Empire
The Success Song
And just for the hell of it, have a Soundtrack petition! Maybe we can just get a CD of all of these some day.